Question
What effects will high humidity levels have on lawns ?

Answer
Hi John G:

>>"...effects will high humidity...?"

Fescue species do not do well with temperatures above 85 degrees F.  High humdity will help this grass type get through the heat.

Lawn grasses can be divided into those recommended for northern climates and those for southern climates,...and some that do ok for intermediate zones or in mixed combinations.  Fescues are best for the cooler climates, but because of their shade-tolerance, the seed is being sold in almost all the USA.

Most Grasses and temperate (north/intermediate)-climate lawn grasses have evolved to be used to periods of drought and frequent periods of transient dry conditions. Some grasses, including fescues, send their roots more than thirty feet into the sub-soil to stay hydrated. Certain other species (St. Augustine, e.g.)  are more accustomed to semi-tropical conditions and therefore more generally humid conditions.
Almost all turf grasses do well with at least 1-3 inches of APPLIED water per-week; but many factors are involved in lawn hydration efficacy such as soil drainage, and so on.  With high humidity, there is less evaporation of moisture from the soil proper and also less moisture loss from the foliage itself (i.e., ...via leaf-surface metabolic transpiration). This means less loss of water from plant tissues, so the vegetation can more easily maintain plant tissue integrity (aka "turgor"), and less watering may be needed with high ambient humidity.  High humidity is therefore generally something favorable for a lush lawn.

 One major draw-back however, is that many fungi are able to prosper with high humidity so there is an increased probable incidence of fungal diseases with turf-grasses and other plants with more water in the air.  With high humidity and cool conditions, fungal problems may be at their worst, so the greens keeper should watch-out for fungus disease out-breaks which can rapidly spread to destroy entire lawn areas when these conditions prevail.  Certain insect problems may also be exaggerated with humid conditions, so be on the watch for these pests as well.
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I Hope this has answered your question(s)!

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___ Jim Gibbs,
    Microbiologist/Botanist